This invention relates to a process for preparing dialkyl carbonates and diols. More specifically the present invention relates to a process for preparing dialkyl carbonates and diols from cyclic carbonates and alcohols with substantially diminished levels of hydroxy alkyl carbonate by-product.
Dialkyl carbonates are important intermediates for the synthesis of fine chemicals, pharmaceuticals and plastics and are useful as synthetic lubricants, solvents, plasticizers and monomers for organic glass and various polymers, including polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transparency, shock resistance and processability.
One method for the production of polycarbonate resin employs phosgene and bisphenol-A as starting materials. However, this method has numerous drawbacks, including the production of corrosive by-products and safety concerns attributable to the use of the highly toxic phosgene. As such, polycarbonate manufacturers have developed non-phosgene methods for polycarbonate production, which use diphenyl carbonate and bisphenol-A as starting materials. Diphenyl carbonate can be prepared from phenol and dimethyl carbonate.
Dimethyl carbonate has a low toxicity and can also be used to replace toxic intermediates, such as phosgene and dimethyl sulphate, in many reactions, such as the preparation of urethanes and isocyanates, the quaternization of amines and the methylation of phenol or naphthols. Moreover, it is not corrosive and it will not produce environmentally damaging by-products. Dimethyl carbonate is also a valuable commercial product finding utility as an organic solvent, an additive for fuels, and in the production of other alkyl and aryl carbonates.
Dimethyl carbonate, as well as other dialkyl carbonates, have traditionally been produced by reacting alcohols with phosgene. These methods have the same problems as methods that use phosgene and bisphenol-A, i.e., the problems of handling phosgene and disposing of phosgene waste materials. Thus, there is a need for commercially viable non-phosgene methods for the production of dimethyl carbonate, as well as other dialkyl carbonates. Non-phosgene methods that have been proposed for producing dialkyl carbonates include the transesterification reaction of alcohols and cyclic carbonates. Most of the proposed methods relate to the use of various catalysts for that reaction. Examples of such proposed catalysts include alkali metals or basic compounds containing alkali metals; tertiary aliphatic amines; thallium compounds; tin alkoxides; alkoxides of zinc, aluminum and titanium; a mixture of a Lewis acid and a nitrogen-containing organic base; phosphine compounds; quaternary phosphonium salts; cyclic amidines; compounds of zirconium, titanium and tin; a quaternary ammonium group-containing strongly basic anion-exchange solid material; a solid catalyst selected from the group consisting of a tertiary amine or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a mixture of an alkali metal with silica, a silicate of an alkaline earth metal and an ammonium ion-exchanged zeolite; and a homogeneous catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a selenium compound.
The catalytic transesterification of a cyclic carbonate with an alcohol involves two equilibrium steps which can generate a hydroxy alkyl carbonate as the reaction intermediate. For example, in the transesterification of ethylene carbonate (EC) with methanol (MeOH), the intermediate which is formed is 2-hydroxyethyl methyl carbonate (HEMC). This two equilibrium step reaction may be represented by the following: 
The amount of hydroxy alkyl carbonate, e.g., HEMC in the case of EC and MeOH, formed is dependent on the type of catalyst employed and reaction conditions used in the process. There can be a significant amount of unreacted hydroxy alkyl carbonate (e.g., HEMC) following the second equilibrium step. Glycols present in the reaction mixture may also react with a cyclic alkyl carbonate to form dihydroxy alkyl carbonates, which may be included as a type of hydroxy alkyl carbonate, and can be decomposed by means discussed herein.
Hydroxy alkyl carbonates are generally highly reactive and thermally unstable organic compounds. Thus, any attempt to separate such compounds from the desired products using typical separation techniques, such as high temperature distillation, would likely cause at least partial decomposition and/or reaction with other organics in the product stream to form by-products and lower reaction yields. In the case of the transesterification of EC with MeOH, possible side reactions include inter- and intra-molecular dehydration of HEMC and dehydration between HEMC and EG.
In addition to lower yields of desired products, the side reactions will likely result in lower purity products or additional capital and operating costs needed to improve product purity.
Thus, there is a need for a process for the production of dialkyl carbonates and diols from cyclic carbonates and alcohols which does not have the above mentioned disadvantages.
According to the present invention, it has now been found that a dialkyl carbonate and diol, and more specifically dimethyl carbonate and ethylene glycol, can be prepared with high yields and high product purity, from a cyclic carbonate and an aliphatic monohydric alcohol.
The process of the present invention involves:
(a) reacting a cyclic carbonate with an aliphatic monohydric alcohol in the presence of a transesterification catalyst in a transesterification reaction zone to provide a crude product stream containing a dialkyl carbonate, diol, hydroxy alkyl carbonate, unreacted cyclic carbonate and unreacted aliphatic monohydric alcohol;
(b) separating a crude dialkyl carbonate product stream containing dialkyl carbonate and unreacted aliphatic monohydric alcohol from the crude product stream;
(c) diminishing, reducing or eliminating the hydroxy alkyl carbonate from the crude product stream; and
(d) recovering the dialkyl carbonate and the diol.
Preferably, the cyclic carbonate of the present invention is of the formula: 
wherein R1 and R2 independently of one another denote a group represented by the formula xe2x80x94(CH2)m, wherein m is an integer from about 1 to about 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C1-C10 alkyl group and a C6-C10 aryl group, wherein R1 and R2 can share the same substituent;
and the aliphatic monohydric alcohol is of the formula:
R3xe2x80x94OH xe2x80x83xe2x80x83(III) 
wherein R3, is an aliphatic C1-C12 hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C1-C10 alkyl group or a C6-C10 aryl group.
In one embodiment, the hydroxy alkyl carbonate is diminished from the crude product stream by:
(i) directing the crude product stream to a conversion reaction zone; and
(ii) converting at least a portion of the hydroxy alkyl carbonate in the conversion reaction zone under conversion conditions to provide additional cyclic carbonate and additional aliphatic monohydric alcohol.
Preferably, substantially all of the hydroxy alkyl carbonate is converted in the conversion reaction zone to additional cyclic carbonate and additional aliphatic monohydric alcohol.
In a preferred embodiment, substantially all of the unreacted aliphatic monohydric alcohol will be removed from the crude product stream, as a result of separating the crude dialkyl carbonate stream.
Preferably, the unreacted aliphatic monohydric alcohol separated from the crude product stream is recycled to the transesterification reactor by:
(i) separating the unreacted aliphatic monohydric alcohol from the crude dialkyl carbonate product stream; and
(ii) recycling the unreacted aliphatic monohydric alcohol to the transesterification reaction.
In yet another embodiment, after separating the crude dialkyl carbonate product stream from the crude product stream and converting the hydroxy alkyl carbonate to additional cyclic carbonate and aliphatic monohydric alcohol, the conversion products will be recycled by:
(i) separating the additional aliphatic monohydric alcohol from the crude product stream;
(ii) recycling the additional aliphatic monohydric alcohol to the transesterification reaction;
(iii) separating the additional cyclic carbonate from the crude product stream; and
(iv) recycling the additional cyclic carbonate to the transesterification reaction.
In a preferred embodiment, the cyclic carbonate is ethylene carbonate, the aliphatic monohydric alcohol is methanol, the dialkyl carbonate is dimethyl carbonate, the diol is ethylene glycol and the hydroxy alkyl carbonate is 2-hydroxyethyl methyl carbonate.
In still another embodiment according to the present invention, a process for the production of a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol comprises: (a) reacting a cyclic carbonate with an aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a crude product stream comprising a dialkyl carbonate, diol, hydroxy alkyl carbonate, unreacted cyclic carbonate and unreacted aliphatic monohydric alcohol; (b) treating the crude product stream under reaction conditions sufficient to at least partially decompose the hydroxy alkyl carbonate, thereby forming a crude dialkyl carbonate product stream; and (c) recovering the dialkyl carbonate and the diol from the crude dialkyl carbonate product stream.
Another process according to the present invention which is capable of producing a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol comprising: (a) reacting a cyclic carbonate with an aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a crude product stream comprising a dialkyl carbonate, diol, hydroxy alkyl carbonate, unreacted cyclic carbonate and unreacted aliphatic monohydric alcohol; (b) separating the dialkyl carbonate and the unreacted aliphatic monohydric alcohol from the crude product stream, thereby forming a crude dialkyl carbonate product stream and a hydroxy alkyl carbonate-rich stream; (c) treating the hydroxyl alkyl carbonate-rich stream under reaction conditions sufficient to at least partially decompose the hydroxy alkyl carbonate, thereby forming a diol-rich stream, a cyclic carbonate rich-stream and a aliphatic monohydric alcohol-rich stream; and (d) recovering the dialkyl carbonate from the crude dialkyl carbonate product stream and the diol from the diol-rich stream.
Yet another process for the production of a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol comprising: (a) in a reactive distillation vessel, reacting a cyclic carbonate with an aliphatic monohydric alcohol in the presence of a transesterification catalyst to provide a crude dialkylcarbonate product stream comprising dialkyl carbonate and unreacted aliphatic monohydric alcohol, and a hydroxyl alkyl carbonate-rich stream; (b) treating the hydroxyl alkyl carbonate-rich stream under reaction conditions sufficient to at least partially decompose the hydroxy alkyl carbonate, thereby forming a diol-rich stream, a cyclic carbonate rich-stream and a aliphatic monohydric alcohol-rich stream; and (c) recovering the dialkyl carbonate and the diol from the crude dialkyl carbonate product stream.
The present invention provides the advantage of diminishing the hydroxy alkyl carbonate from the product stream under mild conditions and producing dialkyl carbonates and diols in relatively high yield and purity from cyclic carbonates and alcohols.
While the present invention has been described and illustrated by reference to particular embodiments, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason then reference should be made solely to the appended claims for purposes of determining the true scope of the present invention.